1. Field of the Invention
The present invention relates to a cylinder apparatus and, particularly to a cylinder apparatus in which a piston is movably inserted into a cylinder bore via a cup seal.
2. Description of Related Art
In a cylinder apparatus, it is known that an annular seal groove is formed on an inner circumferential surface of a cylinder bore into which a piston is movably inserted, and a cup seal for slidably sealing the portion between the cylinder bore and the piston is fitted into the seal groove.
In general, as described in Japanese Patent Unexamined Publication JP-A-2004-231093, the seal groove includes: a seal groove bottom surface extending in a circumferential direction; a bottom-side-surface on a cylinder bore bottom portion side; an opening-side-surface on a cylinder bore opening portion side; and a seal groove opening opened to a piston. Further, the cup seal includes: a base portion disposed on the cylinder bore opening portion side of the seal groove, an inner circumferential lip portion extended from an inner circumferential side of the base portion toward the cylinder bore bottom portion and having an inner circumferential surface which slides relative to an outer circumferential surface of the piston; and an outer circumferential lip portion extended from an outer circumferential side of the base portion also toward the cylinder bore bottom portion and having an outer circumferential surface which abuts against the seal groove bottom surface.
As the seal structure in the cup seal having such a shape, there are known a seal structure in which the tip end side of the inner circumferential lip portion is protruded toward the inner circumference to cause the inner circumferential surface of the tip end of the inner circumferential lip portion to abut against the piston (see, e.g., International Patent Unexamined Publication WO 2004/094208). Further, there also known a seal structure in which the inner circumferential surface of the inner circumferential lip portion is formed in parallel with the outer circumferential surface of the piston to cause the entire inner circumferential surface to abut against the piston (see, e.g., Japanese Patent Unexamined Publication JP-A-2005-186925).
As described in the WO 2004/094208, in the cup seal in which sealing is performed by causing the tip end of the inner circumferential lip portion to abut against the piston, a relief port provided in the piston is positioned at a part close to the tip end of the inner circumferential lip portion in order to reduce an ineffective stroke. However, a flow channel for hydraulic fluid needs to be secured by providing a reduced diameter portion for maintaining the communication between a hydraulic pressure chamber and a reservoir during non-operation of the piston on the outer circumference of the piston or the like. Thus, the problem is encountered that the structure of the cylinder apparatus becomes complicated.
In addition, as described in the JP-A-2005-186925, in the cup seal in which the entire inner circumferential surface of the inner circumferential lip portion is caused to abut against the piston, a seal position for determining a point where pressurization starts at the time of start of the operation of the piston becomes unclear so that there are cases where the ineffective stroke is brought into an unstable state. In this case, the seal position can be clarified by increasing an abutting force of the inner circumferential lip portion against the piston. However, a sliding resistance to the piston is increased so that the problem is encountered that operation feeling is deteriorated.